gunzip changes lastmod time?

[Frank Bax]

On an older box still running 3.5; gunzip/gzip does not change lastmod 
time; but on 4.0 [release] gunzip changes the lastmod time.  What's the 
reason for this change?

$ dmesg > dmesg.txt
$ touch -t 200701010000 dmesg.txt
$ ls -l dmesg.txt
-rw-r--r--  1 fbax  fbax  3797 Jan  1 00:00 dmesg.txt
$ gzip dmesg.txt
$ ls -l dmesg.txt.gz
-rw-r--r--  1 fbax  fbax  1829 Jan  1 00:00 dmesg.txt.gz
$ gunzip dmesg.txt.gz
$ ls -l dmesg.txt
-rw-r--r--  1 fbax  fbax  3797 Apr 19 13:15 dmesg.txt